There's a new open source Cardboard SDK for iOS and Android NDK that offers a streamlined API, improved device compatibility, and built-in viewer profile QR code scanning. A corresponding Unity package (SDK) is planned for a future release. We recommend that all developers actively building for Google Cardboard migrate (iOS, Android NDK) to the new Cardboard SDK.

October 15, 2019 update:

The Daydream View VR headset is no longer available for purchase. However, you can continue to use the existing Google VR SDK to update and distribute your apps to the Google Play Store, and make them available to users in the Daydream app.